1. Field of the Invention
This invention relates to an apparatus for forming images on printing paper such as printers, and more particularly relates to an apparatus for forming images that measures the length of the printing paper while transporting the printing paper.
2. Description of the Related Art
A printer that is directly connected to an imaging device such as a digital still camera and outputs images taken and recorded with the imaging device on relatively small-size printing paper such as post-card size or L size has become widespread recently. With this type of printer, users execute a process of outputting images of their own shooting without any professional help on the printing paper. The printer is requested to print out images without any white space on all sides, that is, with no margins in the periphery of the printing paper, because professionals often print the images taken with silver salt cameras on the printing paper, leaving no margins.
A heat transfer printer disclosed in JP 2003-39759A is equipped with a structure best suited to form images on printing paper without any white space on all sides. In the printer, a plurality of contact members are equipped around a platen roller and are biased towards the outer circumferential surface of the platen roller, the outer circumferential surface being covered with a material having a high coefficient of friction. In an image forming apparatus in which printing paper is fixed on a platen-roller (or a rotating drum and the like) with a fixture such as a clamper or a chuck, it is very difficult to form images with no white space on all sides, whereas this printer, by employing the above-mentioned structure, can roll up printing paper around a platen roller without a fixture to form images with no margins.
When forming images with no white space on all sides, in an image-forming apparatus such as the heat transfer printer, what is known as “overprinting” occurs on the portions not covered with the printing paper. A large amount of ink on the outer circumferential surface of a platen-roller caused by excessive overprinting results in smearing the back face of the printing paper. Furthermore, the adhesion of an over-coating agent on the outer circumferential surface of the platen roller causes malfunction of a sensor provided inside a printer. This is why a conventional image-forming apparatus is configured to control image forming mechanism such as the thermal head so as to perform overprinting appropriately. Besides dealing with a variety of printing paper sizes, an image forming apparatus is designed to control an image forming mechanism so as to perform overprinting appropriately according to the length of the printing paper measured at the time of being transported to the platen roller for the purpose of handling a little variation seen in printing paper size.
Measuring the length of the printing paper in the process of being transported to the platen roller is not only limited to an image forming apparatus with a function of forming images without any white space on all sides, but is widely found in many other image forming apparatuses (cf. JP H5-69608A and JP 2003-335433A). Generally, the length of the printing paper is measured as follows. First, at least the rear end of the printing paper is detected by a contact or non-contact type sensor. And then the rotation angle of the platen roller or the number of revolutions of a driving motor or the like during a period from the predetermined state until when the rear end of the printing paper is detected, or from when the front end of the printing paper is detected until when the rear end is detected are measured. Finally, the length of the printing paper is calculated based on the obtained rotation angle or the like and several constants (e.g. platen roller diameter, etc.).
However, as a printer disclosed in JP 2003-39759A, in an image forming apparatus that rolls up printing paper around a platen roller without any fixture, when measuring the length of the printing paper by detecting the end of the printing paper, the length of the printing paper may not be obtained with a sufficient accuracy. With this type of image forming apparatus, a slip of the printing paper (or its displacement) can occur on the platen roller during transport, when the contact area is not sufficiently ensured between the outer circumferential surface of the platen roller and the printing paper. When a slip occurs, the rotation angle of the platen roller or the number of revolutions of the driving motor or the like employed in calculating the length of the printing paper become greater than in the case where no slip occurs. As a result, the measured length of the printing paper becomes greater than its real length.